Photoperiodism will be investigated in the Syrian hamster with respect to the environmental cues which regulate the annual reproductive cycle. Studies will be performed at several levels including the role of the pineal and of a circadian clock in mediating the reproductive response to light cues. An attempt will be made to identify the pineal antigonadotropin (melatonin?) and to determine the mechanism by which it affects the hypothalami-pituitary axis. Electrical stimulation-recording experiments will be performed in various neural sites (e.g., nervi conarii, superior cervical ganglion, suprachiasmatic nuclei) relevant to the photic entrainment of pineal and reproductive rhythms. Behavioral studies will be directed toward determining how hamsters monitor daylength in the wild. Finally, comparative studies will be performed with two other species of hamsters and with a relatively non-photoperiodic (with respect to annual breeding cycles) species, the laboratory rat.